Rift Valley fever virus (RVFV) is an important mosquito-borne pathogen endemic to Africa and the Arabian Peninsula. In humans, it can cause liver damage, blindness, hemorrhagic fever, and death. Because it kills most young and unborn livestock, the economic consequences of widespread infection can be severe. The virus is carried by a variety of mosquito species, and owing to climate change and global trade, there is reasonable concern that the virus could spread beyond its historic range. There are currently no proven safe, effective vaccines or treatments for RVFV infection. In this application, we propose experiments that will begin to elucidate a novel mechanism by which RVFV evades the cellular antiviral responses to infection. These results could ultimately lead to novel strategies for combatting RVFV and other viral infections. We recently observed a dramatic change in the splicing patterns of several host mRNAs upon infection with RVFV or upon transfection with a plasmid that expresses the viral nucleocapsid protein, N. One gene whose mRNA was subject to a dramatic change in splicing was RIOK3, which was recently implicated as a component of the cellular innate immune response. This alternative splicing results in a mRNA that codes for a severely truncated protein that lacks essential residues for kinase activity. Knockdown of RIOK3 using siRNAs allowed RVFV to replicate to higher titers than in wild-type cells. Therefore, an attractive hypothesis is that the virus evades part of the antiviral cellular response by interfering with splicing of mRNAs that code for RIOK3 and other potentially important antiviral proteins. How does the virus interfere with normal cellular splicing? Using UV-crosslinking followed by immunoprecipitation and deep sequencing (CLIP-seq), we determined that N associates strongly with C/D-box snoRNAs (SNORDS) in infected cells. The usual job of SNORDs is to guide methyltransferases to specific residues within the rRNA during ribosome assembly, but it has been shown recently that SNORDs, when separated from the proteins that constitute the rRNA methylation complex, are also capable of altering the splicing patterns of specific pre-mRNAs. We hypothesize that N, by competitive binding to SNORDs, alters their localization and their function, which contributes to changes in mRNA splicing patterns. To test this hypothesis, we will carry out biochemical and bioinformatics experiments to determine whether N alters SNORD complex formation and localization, and whether specific SNORDs are involved in changing the splicing patterns of specific pre-mRNAs. The overall objective of this proposal is to test a novel but plausible mechanism for the N-mediated change in splicing patterns. The data generated here will establish the proof of principle for this novel strategy of viral escape from host antiviral defenses and will form the basis for a larger-scale research project to fully elucidate the underlying molecular mechanisms.